As oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as “oil wells”) become deeper, higher strength is required for oil-well steel pipes. Conventionally, oil-well steel pipes of 80 ksi grade (yield strength is 80 to 95 ksi, that is, 551 to 654 MPa), and of 95 ksi grade (yield strength is 95 to 110 ksi, that is, 654 to 758 MPa) have been widely used. However, in recent years, oil-well steel pipes of 110 ksi grade (yield strength is 110 to 125 ksi, that is, 758 to 862 MPa) have been started to be used.
Many of deep wells contain hydrogen sulfide which has corrosiveness. For that reason, an oil-well steel pipe for use in deep wells is required to have not only high strength but also sulfide stress cracking resistance (hereinafter referred to as SSC resistance).
Conventionally, as a measure to improve the SSC resistance of an oil-well steel pipe of 95 to 110 ksi classes, there is known a method of cleaning steel or refining steel structure. In the case of the steel proposed in Japanese Patent Application Publication No. 62-253720 (Patent Literature 1), impurities such as Mn and P are reduced to increase the level of cleanliness of steel, thereby improving the SSC resistance of steel. The steel proposed in Japanese Patent Application Publication No. 59-232220 (Patent Literature 2) is subjected to quenching twice to refine crystal grains, thereby improving the SSC resistance of steel.
However, the SSC resistance of steel material significantly deteriorates as the strength of steel material increases. Therefore, for practical oil-well steel pipes, a stable production of an oil-well pipe of 120 ksi class (yield strength is 827 MPa or more) having the SSC resistance which can endure the standard condition (1 atm H2S environment) of the constant load test of NACE TM0177 method A has not been realized yet.
Under the background described above, an attempt has been made to use high-C low alloy steel having a C content of 0.35% or more, which has not been put into practical use, as an oil-well pipe to achieve high strength.
The oil-well steel pipe disclosed in Japanese Patent Application Publication No. 2006-265657 (Patent Literature 3) is produced by subjecting low alloy steel containing C: 0.30 to 0.60%, Cr+Mo: 1.5 to 3.0% (Mo is 0.5% or more), and others to tempering after oil-cooling quenching or austempering. This literature describes that the above described production method allows to suppress quench cracking which is likely to occur during quenching of high-C low alloy steel, thereby to obtain an oil-well steel or oil-well steel pipe, which has excellent SSC resistance.
The oil-well steel disclosed in Japanese Patent No. 5333700 (Patent Literature 4) contains C: 0.56 to 1.00% and Mo: 0.40 to 1.00%, and exhibits not more than 0.50 deg of a half-peak width of (211) crystal plane obtained by X-ray diffractometry, and yield strength of 862 MPa or more. This literature describes that SSC resistance is improved by spheroidizing of grain boundary carbides, and the spheroidizing of carbides during high temperature tempering is further facilitated by increasing the C content. Patent Literature 4 also proposes a method of limiting a cooling rate during quenching, or temporarily stopping cooling during quenching and performing isothermal treatment to hold in a range of more than 100° C. to 300° C., in order to suppress quench cracking attributable to a high-C alloy.
The steel for oil-well pipe disclosed in International Application Publication No. WO2013/191131 (Patent Literature 5) contains C: more than 0.35% to 1.00%, Mo: more than 1.0% to 10%, and others in which the product of C content and Mo content is 0.6 or more. Further in the above described steel for oil-well pipe, the number of M2C carbide which has a circle equivalent diameter of 1 nm or more, and has a hexagonal structure is 5 or more per 1 μm2, and the half-peak width of the (211) crystal plane and the C concentration satisfy a specific relationship. In addition, the above described steel for oil-well pipe has yield strength of 758 MPa or more. In Patent Literature 5, a quenching method similar to that in Patent Literature 4 is adopted.
However, even with the techniques of Patent Literatures 3 to 5, it is difficult to obtain excellent SSC resistance and high strength in a thick-wall oil-well steel pipe, more specifically in an oil-well steel pipe having a wall thickness of 40 mm or more. In particular, in a thick-wall oil-well steel pipe, it is difficult to obtain high strength and reduced variation in strength in the wall-thickness direction.